TEST 3: Verifying The Ignition Coil Is Getting Power
So far, your troubleshooting efforts have confirmed two very important things:
One: That the ignition coil's high tension wire is not the cause of the no spark-no-start problem.
Two: That the ignition coil's tower is not firing off spark.
The next couple of tests are to see if the ignition coil is getting fed with power (12 Volts) on the DK GRN/BLK wire and is receiving a switching signal on the GRY/WHT wire of its connector.
In this test step, you'll check for these 12 Volts on the DK GRN/BLK wire either using a multimeter or a 12 V DC test light.
IMPORTANT: Depending on your Jeep's model and year, the wire that feeds the ignition coil with power will be a DK GRN/BLK (dark green w/ black stripe) wire or a DK GRN/ORG (dark green w/ orange stripe) wire or a DK GRN/RED (dark green w/ red stripe) wire.
The following wiring diagrams will help you check the color of the wires of the igntion coil's connector:
- Ignition System Wiring Diagram (1993-1995 4.0L Jeep Grand Cherokee).
- Ignition System Wiring Diagram (1996-1998 4.0L Jeep Grand Cherokee).
Alright, this is what you'll need to do:
- 1
Place your multimeter in Volts DC mode.
- 2
With the red multimeter test lead probe the DK GRN/BLK (or DK GRN/ORG or DK GRN/RED) wire of the ignition coil's harness connector.
- 3
Connect the black multimeter test lead on the battery's negative (-) post.
- 4
Have a helper crank the engine. This is important, because the ignition coil will only get power when the engine is either cranking or running.
- 5
Your multimeter should register 10 to 12 Volts DC.
Let's take a look at what your results mean:
CASE 1: The multimeter registered 10 to 12 Volts. This is the correct test result.
The next step is to check that the ignition coil is getting a switching signal from the Powertrain Control Module (PCM). Go to: TEST 4: Testing The Switching Signal.
CASE 2: The multimeter DID NOT register 10 to 12 Volts. Double check your multimeter connections and repeat the test.
If your multimeter results still do not indicate 12 Volts, then ignition coil is not fried and not the cause of the no spark no-start problem, since without power, it won't work.
The two most likely causes for this missing voltage are: 1.) The Auto Shut Down (ASD) Relay is bad or 2.) the crankshaft position sensor is bad.
Although it's beyond the scope of this article to find the cause of these missing 12 Volts, resolving this issue will solve the no-spark no-start issue.
TEST 4: Testing The Switching Signal
The ignition coil is activated, to fire spark, by your Jeep's PCM.
In a nutshell: The PCM accomplishes this by interrupting the ignition coil's path to Ground on the grey with white stripe (GRY/WHT) wire of the coil's harness connector. This interruption causes the ignition coil to fire off a spark.
This process of switching the ignition coil's primary voltage (this is the fancy name for the 12 Volts the coil gets and that you checked in TEST 3) on and off can be very easily checked with a simple 12 volt automotive test light.
IMPORTANT: Depending on the model and year of your specific 4.0L Jeep, the wire that feeds the ignition coil with the switching signal will be a: GRY (grey) wire, or a GRY/WHT (grey w/ white stripe) wire, or a BLK/GRY (black w/ grey stripe) wire.
The following wiring diagrams will help you check the color of the wires of the igntion coil's connector:
- Ignition System Wiring Diagram (1993-1995 4.0L Jeep Grand Cherokee).
- Ignition System Wiring Diagram (1996-1998 4.0L Jeep Grand Cherokee).
These are the test steps:
- 1
Reconnect the high tension wire to the ignition coil and the distributor cap, if you haven't done so.
- 2
Probe the GRY/WHT (or GRY or BLK/GRY) wire of the ignition coil's connector with your 12 Volt test light's metal probe.
- 3
Connect the alligator clip (of the 12 V test light) to the positive (+) battery terminal.
The ignition coil can be connected to its electrical connector or not. - 4
When ready, have your helper crank the engine while you observe and hold the test light in place.
- 5
Your 12 Volt test light will:
1.) Flash ON and OFF the whole time the engine is cranking.
2.) It will not flash ON and OFF as your Jeep's engine cranks.
Let's take a look at what your results mean:
CASE 1: The test light flashed ON and OFF. This is the correct and expected test result.
You can correctly conclude that the ignition coil is bad and needs to be replace if you have:
- Confirmed that the ignition coil's high tension wire is not sparking (TEST 1).
- Confirmed that the ignition coil's tower is not sparking (TEST 2).
- Confirmed that the ignition coil is getting 10 to 12 Volts DC (TEST 3).
- Confirmed that the ignition coil is getting the switching signal (this test section).
CASE 2: The test light DID NOT flash ON and OFF. This test result exonerates the ignition coil, since without this switching signal, it won't spark.
The most likely causes of this missing switching signal are: a bad crankshaft position (CKP) sensor, an open-circuit (or a short) in the wiring between the PCM and the ignition coil's connector, or the PCM (Powertrain Control Module = fuel injection computer) is bad (although this is a rare thing).
Altho' it's beyond the scope of this article to test the wiring or the PCM or the crank sensor, you have at least eliminated the ignition coil as bad.
The following tutorial will help you test the crankshaft position (CKP) sensor:
More 4.0L Jeep Diagnostic Test Tutorials
You can find a complete list of 4.0L Jeep wiring diagrams and diagnostic test tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- How To Troubleshoot A No Start Condition (Jeep 4.0L).
- How To Test The MAP Sensor (1997-2003 Jeep 4.0L).
- Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
- How To Troubleshoot A Bad Fuel Injector (Jeep 4.0L).
- How To Test Engine Compression (Jeep 4.0L).
- How To Test The Crankshaft Position Sensor (1994-1996 4.0L Jeep).
If this info saved the day, buy me a beer!
